Variable trim resistor



March 17, 1959 Q AMSLER VARIABLE TRIM RESISTOR Filed Sept. 24, 1956'ff/1y.

INVENTOR. CL V05 ,4A/.SL59 7- BY United States Patent O 2,878,350VARIABLE TRIM RESISTOR Clyde Amsler, Santa Monica Canyon, Calif.,assignor to Hycon Mfg. Company, Pasadena, Calif., a corporation ofDelaware My invention relates to variable resistors and has particularreference to a miniature resistor construction wherein preciseadjustment can be obtained by rotating an outer shell relative to aninner member.

The production of precise electronic equipment inv ariably depends uponprecise components of which reslstors constitute a principal part. Fixedresistors frequently are so inaccurately constructed as to be of limitedvalue and adjustment to some other circuit component or condition isfrequently desired. Variable resistors accordingly constitute adesirable component for precision circuits. Such resistors are referredto as trim resistors, that is, adjustable to a precise value as comparedto resistors that are intended to be variable during operation.

While variable resistors have been used for decades, they are of limitedutility, generally being of considerable bulk and requiring manualmanipulation to adjust them to the desired value. The usual externalslidersv are a short circuit hazard and the physical assembly of thesevariable resistors is diticult in miniature circuits.

I have devised an adjustable resistor or trim resistor that is compactlyconstructed and simple to adjust. A simple rotation of an inner shelleffects minute and precise changes in the eiective resistance values.The design lends itself to miniature trim resistors, making practicalhigh precision adjustable resistors that are an inch or less in length.

It is therefore a general object of the invention to provide improvedwire wound resistors that are precisely adjustable in resistance.

Another object is to provide a resistor that is adjustable in resistanceby rotation of an outer shell with respect to an inner member.

A further object is to provide a variable resistor that is wire woundand the wiper element is threaded upon internal threads on an outercase.

Other objects and advantages of my invention will be apparent in thefollowing descriptions and claims considered together with theaccompanying drawings forming an integral part of this specification andin which:

Fig. l is a sectional view through one form of embodi ment of theinvention wherein the wiper is threaded to the outer case.

Fig. 2 is a sectional view along the line II--II of Fig. 1.

Fig. 3 is a sectional view through another embodiment of my inventionwherein the variable resistor is disposed within the central opening ofa toroidal winding.

Fig. 4 is a sectional view along the line IV-IV of Fig. 3.

Fig. 5 is a sectional view through a third modification of the inventionwherein differential threads advance a winding with respect to a wiper.

Fig. 6 is an end view taken along the line VI--VI of Fig. 5.

Fig. 7 is a sectional view through another modied form of the invention.

Referring to Figs. 1 and 2 the resistor is referred to generally bynumeral 10 and may have a first member in the form of an outer shell 11,a second member in the form of a wiper or contact 13, preferably formedof electrically conductive material. The wiper 13 may be generallytubular in shape and may be externally threaded to engage internalthreads on the shell 11 so that the wiper may be moved inwardly andoutwardly of the shell by relative rotation. A loop or terminal 14 maybe soldered or otherwise electrically connected to the wiper 13.

The resistor core 12 may be secured to the shell 11 in any suitablemanner and there is illustrated a terminal eye 16 projecting through theclosed end of the shell 11 and Ithe resistor core 12 may be mechanicallysecured to this terminal eye by threads or other mechanical connection.In the form illustrated in Fig. l the conductor core 12 may be formed ofinsulating material about which may be wrapped a helically woundresistor wire 17, one end of which may be electrically connected to theterminal eye 16. To stabilize the resistor core 12 within the shell 11 atubular sleeve 18 may be inserted between the wiper 13 and the wirewindings 17.

In operation the device of Figs. l and 2 is adjusted for resistance byrelative rotation between the wiper 13 and the outer shell 11. The wiper13 may have one or more inwardly projecting wiper points 13a, preferablyon one end thereof, which may engage the resistance wire 17 disposedabout the core 12. As the wiper 13 moves to the left in Fig. 1 it willcontact an increasing length of the resistance wire 17 increasing theelectrical resistance between the terminals 16 and 14. In this fashion avery precise resistance adjustment may be obtained. lf the pitch of thewire windings 17 is the same as the threads between the shell 11 and thewiper 13 there will be no sliding of the wiper transverse to theresistance wire and the resistance adjustment will be most precise inythat it operates along the length of a wire rather than jumping fromone wire winding to the adjacent wire winding.

Illustrated in Figs. 3 and 4 is a trim resistor embodying the inventionin combination with a winding which may be either an inductive orresistance winding, depending upon the application. A spool 21 may havewire windings 22 disposed thereon and may be closed if desired by acylindrical cover 23. One end of the wind# ing 22 may be electricallyconnected to a terminal 24 and the other end may project toward thecenter of the spool as at 22a. Disposed within the center of the spool21 may be an externally threaded wiper 26 having a pair of inwardlyprojecting points 26a to engage a resistance wire 27 wrapped about aresistor core 28. This resistor core 28 may be of electricallyconductive material if desired so that it may be riveted within thespool 21, or may be of insulating material such as plastic which isheaded with heat so as to define a rivet head 29 to retain the corewithin the spool. The free cantilever end of the core 28 may besupported within the tubular wiper 26 by a close fitting cap 31positioned on the end of the resistor core. The wiper 26 may be slottedat its outer end as at 32 to permit screwdriver rotation and may have ahole 33 therethrough to act as an electrical terminal.

The operation of the device of Figs. 3 and 4 is similar to that of Fig.l in that relative rotation of the wiper 26 and the spool 21 causes thewiper to advance along the windings 27 to vary the effective length ofthe resistance wire 27.

Illustrated in Figs. 5 and 6 is an embodiment of the invention whereinextremely precise resistance adjustments are obtained by means ofdiierential screws. In those gures an outer shell 36 may be cylindricalin shape and may have the bore through one end slotted lengthwise as at37. The other end of the bore may have inserted therein a resistancecore member 38 that is not only rotatable within-the shell but ismovable lengthwise of the shell. This resistance core 38 may be formedof conductive material and may have a reduced diameter shoulder on whichis disposed a layer of insulating material 39 either as a iilm of paintoras a ribbon of solid material. Helically wound on this insulator 39may'be a continuous resistance wire d havingV one end 41a' electricallyconnected to the resistance core 3E.

Inserted in the left end of the shell of Fig. may be a rotatable cap 42of conductor material having a screwdriver slot i3 and a pair of holeswhich receive driving pins 44 passing through the rotatable resistancecore 38. These pins may likewise be of'conductor material. The cap 42may beV retained in the shell 38 in aV suitable manner as by the snapring 46 and may be electrically contacted at al1 times by the terminalring 47.

Projecting centrally inwardly from the cap 42 may be a threaded stem 4Sengaging internal threads on a tubular member 49 iitted in the other endof shell 36. This tubular member 49 may have a transverse pin 51inserted therethrough which rides in the slot 37' to prevent rotation ofthat tube. Accordingly, rotation of the stem 4S will reciprocate thethreaded tube 49 inwardly and outwardly of the shell 36.

The tube 49 is also externally threaded as at 52 to engage internalthreads 53 on the resistor core 38. Therefore, as the tube 49reciprocates it tends to reciprocate with it the resistor core 38.However, the resistor core 3S is also rotatable in the same pitchdirection as the stem 48 so as to counteract the reciprocating action ofthe tube 49. By making the threads SZ and S3 of a slightly differentpitch from those on the stem 48, a dierential threading action isobtained so that the actual reciprocation of the resistor core 38 is buta very tiny fraction per revolution of the cap 42 of the pitch dimensionof the threads on stem 4S. A terminal member 54 may project through thewall of the shell 36 to contact the resistance wires 41 to therebycreate a conductive path through the resistance wires 41 from theterminal 47. By making the pitch of the resistance wires 41 equal thatof the differential between the'threads, very precise resistances can beobtained because of the single wiper 54.

The operation of the device of Figs. 5 and 6 is eiected by rotating thecap 42 which, in turn, causes the same degree of rotation of the stem d8and the resistor clore 38. The differential between the threads 52 andthose of the stem 48 causes a diierential reciprocation of the resistorcore 38 with respect to the shell 36 and the longitudinally movable tube49.

Illustrated in Fig. 7 is a modication of the invention wherein theresistor core is threaded within the outer shell. Accordingly, an outercylindrical shell 56 may have a reduced diameter end plate 57 threadedto receive an externally threaded resistor core 58 having a terminal eye59 formed on one end thereof. This resistor core 53 may have a reduceddiameter end portion 58a having a suitable insulating film thereon uponwhich may be wrapped or wound a resistance wire 6l. This wire may beelectrically connected to the resistor core 58 as at 61a. A tubularwiper 62 may be inserted within the shell 56 and may have wiper points62a engaging the resistance wire 61. A terminal 63 may be electricallyconnectedl to the wiper 62.

The operation of the device of Fig. 7 is accomplished by rotating thecore 58 relative to the shell 62 moving the resistance windings 61relative to the wiper 62.

While I have described my invention with respect to speciic embodimentsthereof,.I do not limit myself to these embodiments,nor otherwise, butclaim all modiiications and variations that come within the true spiritand scope oli` the invention.

lclaim:

l. A variable resistor of high adjustment precision comprising an outerinsulating shell;l a screw mounted for rotation within the shell; a corerotatable within the shell; a resistance wire wound onthe core; meansdriving the core in rotation with the screw and providing free movementof the core lengthwise of the screw; diterential thread meansinterconnecting the screw and shell and core; al wiper mounted' on theshell and contacting the resistancewire; and a terminal connected to oneendl of thev resistance "wire, whereby relative rotation of the screwand shell cause dierential movement of the core toy give every exactadjustment of the wiper on the wire.

2,. A resistor that is adjustable in resistance comprising4 a rst memberforming an outer insulator shell; a second member forming a resistorcore disposed within the shell; a third member of conductive materialforming a wiper disposed between the core and the shell' and connectedto a terminal; resistance wire disposedV on said core and connected to aterminal at one end; and difierential thread means interengaging atleast two of said three members so that-relative rotation between saidtwo members will reciprocate the wiper member relative to thecorejmemben'so that a variable resistance is developed between* theterminals.

y3. A variable resistor of'highradjustment precision comprising an outervshell of insulating material; an internally and-- externally threadedtubular member disposed withinl the shell; means kmounting said tubularthreaded member for reciprocation within the shell and preventingrelative rotation with respect to the shell; a core disposed within theshell; a rotatable cap formed of electrical conductor material anddisposed within the shell at one end thereof; a tubular core slidinglymounted on the core for reciprocation within the shell and rof tatablydriven by the cap and disposed within the shell and internally threadedto engage the external threads on the tubular memberya screw secured tothe cap c oaxially with the tubular member and threadedly engaging theinternal' threads on that tubular member; resistance Wire wound on theexterior of the core and having one end electrically connected to thecap; and a wiper connected' to the shelland engaging the resistance wireand having one end projecting lbeyond the shell to act as a terminal,characterized by the internal threads on the tubular member being of adiierent pitch than the external threads on the tubular member, so thatrotation of the cap gives rise to a dilerential movement of the core togive precise resistance settings.

References Cited in the iile of this patent UNITED STATES PATENTS2,481,682 Payne Sept. 13, 1949 2,704,800 Graybeal et al Mar. 22, 19552,778,908 Altieri Jan. 22, 1957

